Spectral shaping for multiband OFDM transmitters with time spreading

ABSTRACT

A method of shaping an orthogonal frequency division multiplexing (OFDM) signal spectrum of a transmitted signal is disclosed. An input signal including an input component is received and a first instance of the input component is generated. The method also includes determining that a second instance of the input component is to be different than the first instance. The second instance of the input component that is different from the first instance is generated. An output signal to be transmitted is generated and includes the first instance and the second instance of the input component.

CROSS REFERENCE TO OTHER APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 60/560,948 (Attorney Docket No. AIELP024+) entitled SPECTRAL SHAPINGFOR MULTIBAND OFDM TRANSMITTERS WITH TIME SPREADING filed Apr. 8, 2004which is incorporated herein by reference for all purposes.

Co-pending U.S. patent application Ser. No. 10/960,431 (Attorney DocketNo. AIELP015) entitled SPECTRAL SHAPING IN MULTIBAND OFDM TRANSMITTERfiled Oct. 6, 2004 is incorporated herein by reference for all purposes;co-pending U.S. patent application Ser. No. 10/960,430 (Attorney DocketNo. AIELP030) entitled SPECTRAL SHAPING IN MULTIBAND OFDM TRANSMITTERWITH CLIPPING filed Oct. 6, 2004 is incorporated herein by reference forall purposes; and co-pending U.S. patent application Ser. No. 10/960,432(Attorney Docket No. AIELP031) entitled SPECTRAL SHAPING IN MULTIBANDOFDM TRANSMITTER WITH PHASE SHIFT filed Oct. 6, 2004 is incorporatedherein by reference for all purposes.

This application is a continuation of co-pending U.S. patent applicationSer. No. 11/099,224 (Attorney Docket No. AIELP024), entitled SPECTRALSHAPING FOR MULTIBAND OFDM TRANSMITTERS WITH TIME SPREADING filed Apr.4, 2005 which is incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

Multiband orthogonal frequency division multiplexing (MB-OFDM) is amodulation technique used in some wireless communication systems such asultra-wideband (UWB). The MB-OFDM modulation technique combines OFDMmodulation with frequency hopping. It is a modulation technique suitablefor devices designed to comply with Federal Communications Commission(FCC) regulations relating to UWB devices.

Unlike most other wireless systems in which the transmit power limit istypically set with respect to the total power integrated over the entiresignal band, UWB devices are allowed to operate within a relatively widefrequency band provided that two criteria are met. First, the occupiedbandwidth is required to meet a predefined minimum. Second, the radiatedpower measured over an integrating bandwidth anywhere within the signalband is required to be less than a predefined maximum. According to thecurrent regulations, UWB devices are allowed to operate in the frequencyband between 3.1 to 10.6 GHz. The occupied bandwidth is required to meeta minimum of 500 MHz and the radiated power, when measured over abandwidth of 1 MHz anywhere within the signal band, is required to beless than −41.3 dBm.

Since in UWB the integrating bandwidth (1 MHz) is much smaller than thebandwidth of the UWB signal itself (500 MHz), the shape of the spectrumis an important issue. In order to maximize the output power of aMB-OFDM transmitter, the spectrum of the generated signal should be madeas flat as possible. FIG. 1A is a diagram illustrating a frequencyspectrum of an ideal UWB signal. In practice, factors such as D/Aconverter pulse shape, non-ideal filter characteristics, componentvariations and data characteristics tend to affect the shape of thespectrum. FIG. 1B is a diagram illustrating the frequency spectrum of atypical UWB signal generated by an existing device. There are peaks andvariations in the frequency spectrum. The transmit power is typicallylimited by the largest peak in the signal spectrum. It would bedesirable to have a UWB MB-OFDM transmitter design that would generate aflat output spectrum over the operating frequency range of thetransmitter.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1A is a diagram illustrating a frequency spectrum of an ideal UWBsignal.

FIG. 1B is a diagram illustrating the frequency spectrum of a typicalUWB signal generated by an existing device.

FIG. 1C is a diagram illustrating the transmission of an OFDM-packetusing multiple frequency hopping bands.

FIG. 2A is a diagram illustrating the frequency spectrum of three hopbands generated by some transmitter embodiments.

FIG. 2B is a diagram illustrating the frequency spectrum of three hopbands generated by a transmitter embodiment that compensates the effectsshown in FIG. 2A.

FIG. 3 is a block diagram illustrating an OFDM transmitter embodiment.

FIG. 4A is a diagram illustrating a frequency spectrum of a set ofuncompensated sub-carriers within a frequency hopping band.

FIG. 4B is a diagram illustrating the frequency spectrum of thesub-carriers after the gain factors are applied.

FIG. 5 is a block diagram illustrating an OFDM transmitter embodimentthat adjusts the sub-carrier amplitudes.

FIG. 6A is a diagram illustrating the frequency spectrum of a signal.

FIG. 6B is a diagram illustrating the clipped frequency spectrum.

FIG. 7 is a flowchart illustrating a frequency clipping processaccording to some embodiments.

FIG. 8 is a block diagram illustrating another OFDM transmitterembodiment.

FIG. 9 is a block diagram illustrating another OFDM transmitterembodiment that implements the phase shift.

FIG. 10 is a diagram illustrating a transmitter embodiment that includesseveral spectrum shaping components.

FIG. 11 is a diagram illustrating an embodiment of a frequency hoppingtransmission that employs time spreading.

FIG. 12 is a flowchart illustrating an embodiment of a process fortransmitting a subset of OFDM symbols with different instances.

FIG. 13 illustrates an embodiment of a shifted pseudo random sequenceused to select instances of OFDM symbols to modify.

FIG. 14A illustrates an embodiment of a transmitter which implementsspectral shaping using time spreading in the time domain.

FIG. 14B illustrates an embodiment of a transmitter which implementsspectral shaping using time spreading in the frequency domain.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess, an apparatus, a system, a composition of matter, a computerreadable medium such as a computer readable storage medium or a computernetwork wherein program instructions are sent over optical or electroniccommunication links. In this specification, these implementations, orany other form that the invention may take, may be referred to astechniques. A component such as a processor or a memory described asbeing configured to perform a task includes both a general componentthat is temporarily configured to perform the task at a given time or aspecific component that is manufactured to perform the task. In general,the order of the steps of disclosed processes may be altered within thescope of the invention.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

Spectrum shaping techniques for transmitting OFDM signals are disclosed.In some embodiments, a band gain control is used to determine a bandgain for a selected band associated with the signal to be transmitted.In some embodiments, a sub-carrier amplitude control is used to apply again factor to each of the sub-carrier frequency components of themodulated signal. In some embodiments, modified synchronizationsequences are used to reduce peaks in the frequency spectrum. In someembodiments, a random phase shifter introduces random or pseudorandomphase shifts to the signal to reduce repetitive patterns in the signaland achieve flatter frequency spectrum. Combinations of these techniquescan be used in various embodiments. For purposes of illustration,spectrum shaping of frequency hopping OFDM signals is discussed indetail below.

In some MB-OFDM systems, multiple frequency hopping bands (also referredto as hop bands or transmission bands) are used to transmit OFDM symbolsto avoid symbol collision. An OFDM symbol waveform includes a number ofmodulated carrier waveforms, referred to as sub-carriers. Eachsub-carrier is used to carry one data symbol, encoded as a phase shiftor a combination of amplitude shift and phase shift. In someembodiments, the sub-carrier frequency spacing is approximately equal tothe inverse of the OFDM symbol duration, which means that thesub-carrier waveforms partly overlap in the frequency domain. FIG. 1C isa diagram illustrating the transmission of an OFDM packet using multiplefrequency hopping bands. In the example shown, each rectanglecorresponds to a synchronization symbol or an OFDM symbol. The initialpart of the packet, referred to as the synchronization preamble,includes a sequence of identical packet synchronization (PS) symbols,followed by a small number of frame synchronization (FS) symbols. Thesynchronization symbols are used to aid the receiver in synchronizing tothe received signal. In the example shown, the synchronization symbols,include a specific sequence of binary phase shift keying (BPSK) symbolsknown as the synchronization sequence. The PS and FS symbols areidentical except for a phase shift of 180°, making them easilydistinguishable to the receiver. Detecting the location of the FSsymbols allows the receiver to determine the boundary between thesynchronization preamble and the header/data portion of the packet.

For the purpose of illustration, three frequency hopping bands are usedin the examples below, although any number of frequency hopping bandsmay be used as appropriate. FIG. 2A is a diagram illustrating thefrequency spectrum of three hop bands generated by some transmitterembodiments. In this example, the frequency spectrum that includes hopbands 202, 204 and 206 is uneven. The unevenness of the frequencyspectrum is sometimes due to component gain difference (i.e. the gaindifference introduced by transmitter components such as mixers,amplifiers, filters and the antenna). Variations in the manufacturingprocess and changes in the operating environment are some additionalfactors that may contribute to the gain difference.

FIG. 2B is a diagram illustrating the frequency spectrum of three hopbands generated by a transmitter embodiment that compensates the effectsshown in FIG. 2A. In this example, frequency hopping bands 252, 254 and256 each has a corresponding band gain used to compensate and adjust thesignal strength to achieve a relatively flat frequency spectrum 260. Theadjustment is made by determining the frequency hopping band associatedwith the signal to be transmitted, determining the band gain thatcorresponds to the frequency hopping band and applying the band gain tothe signal. The band gains are determined during the design process ofthe transmitter in some embodiments to correct any systematic gaindeviations for different hop bands. In some embodiments, anuncompensated output signal is measured to supply feedback informationused to determine the band gain values and achieve the desired frequencyspectrum characteristics. The feedback technique can be used during themanufacturing process, during the transmission operations of thetransmitter or both.

FIG. 3 is a block diagram illustrating an OFDM transmitter embodiment.In this example, transmitter 300 outputs a signal with a gaincompensated frequency spectrum similar to 260. Data bits are received onmedium access control (MAC) interface 302 and then encoded by a forwarderror correction (FEC) encoder 304. In some embodiments, the encodedbits are optionally punctured, interleaved and repeated to providebetter protection against multipath and interference. The bits are thenmapped to modulation symbols by a symbol modulator 306. Quadrature PhaseShift Keying (QPSK) or other appropriate modulation scheme may be used.The modulated symbols such as QPSK symbols are also referred to assub-carriers. Optionally, pilot tone inserter 306 adds pilot tones tothe modulated symbols. An inverse Fast Fourier Transform (IFFT)component 308 is used to transform blocks of symbols from frequencydomain into a time domain waveform (also referred to as an OFDM symbol).A synchronization preamble that includes repeated PS and FSsynchronization symbols is added to the beginning portion of each datapacket by preamble inserter 310. Alternatively, the preamble may beinserted before the IFFT (i.e., in the frequency domain). A guardinterval and a cyclic prefix or zero prefix are added to the OFDM symbolby prefix and guard inserter 312.

In this example, band gain control 314 applies a time varying band gainfactor on its input to counter the effects of gain variations indifferent hop bands to achieve a more uniform frequency spectrum.Depending on the value of the gain factor that is applied, the signalgets amplified, attenuated or remains unchanged as appropriate. Bandgain control 314 is controlled by a hop timing signal and a band selectsignal. Gain values that correspond to different hop bands are stored ina lookup table or other appropriate storage. The hop timing signaldetermines when the band gain factor should change according to thetiming of the OFDM symbol generation. The band select signal determinesthe value of the band gain factor used for a given hop band. In someembodiments, signal strength is measured during operation and anappropriate gain is determined according to the measurement.

The inphase (I) and quadrature (Q) components of the gain compensatedbaseband OFDM signal are converted from digital to analog by digital toanalog converters (DACs) 316 and 318, respectively. The analog signalsare sent to a radio transmitter 320 to be up-converted to the desiredcarrier frequency, amplified and then transmitted via antenna 324. Thelocal oscillator (LO) signal used by radio 320 is generated by frequencysynthesizer 322, which is also controlled by the control signals.Frequency synthesizer 322 has the ability to switch its output frequencyat the start of each OFDM symbol period so that different transmittedOFDM symbols may occupy different hop bands. In some cases, the LOfrequency is switched every symbol period. In other cases, the LOfrequency remains the same for several symbol periods before it isswitched again. It is also possible that the LO frequency is neverswitched during the transmission of an entire packet. The timing of thefrequency switch is controlled by the hop timing signal. The appropriateLO frequency to be synthesized for a given symbol period is determinedby the band select signal.

Variations in frequency spectrum exist among frequency hopping bands aswell as among sub-carriers. FIG. 4A is a diagram illustrating afrequency spectrum of a set of uncompensated sub-carriers within afrequency hopping band. Ideally, the sub-carriers should have equalamplitude and form a flat frequency envelope. In practice, uncompensatedsub-carriers such as 402, 404 and 406 have different amplitudes and forma frequency envelope 400 with amplitude variations. In the diagramshown, the signal amplitudes near the edges of the envelope aresignificantly weaker than those near the center of the envelope. Theamplitude variations are partly attributed to the transmitter's DACs,which introduce a sin(x)/x shaping of the signal spectrum and cause thereduction in signal amplitude near the band edges. The various filtersin the transmitter's signal path have a similar effect as the DAC. Insome embodiments. these filters also cause a ripple in the signalspectrum.

In some embodiments, the effects of the DACs, the filters as well asother components are offset using gain compensation. A plurality ofsub-carrier gain factors are applied to the uncompensated sub-carriersto make the amplitudes of the resulting gain compensated sub-carrierapproximately equal. Depending on the value of the gain factor used, thecorresponding uncompensated sub-carrier amplitude may be amplified,attenuated or unchanged. FIG. 4B is a diagram illustrating the frequencyspectrum of the sub-carriers after the gain factors are applied. Eachsub-carrier is multiplied with an appropriate sub-carrier gain factor.The values of the gain factors are selected such that when multipliedwith corresponding sub-carriers in signal 415, the resulting compensatedsub-carriers have approximately the same amplitude. For example, thesub-carriers near the band edges receive greater gain boost than thesub-carriers near the band center. The resulting sub-carrier envelope430 is substantially more even compared to 406. In some embodiments, theamplitudes of uncompensated sub-carrier frequency components aremeasured to supply feedback information used to determine thesub-carrier gain factors. The measurement may take place during themanufacturing process, during the transmission operations of thetransmitter or both.

FIG. 5 is a block diagram illustrating an OFDM transmitter embodimentthat adjusts the sub-carrier amplitudes. In this example, likecomponents of transmitter 500 and transmitter 300 perform likefunctions. Transmitter 500 additionally includes a sub-carrier gaincontrol 502 that adjusts the amplitudes of the sub-carriers by applyingappropriate gain factors to the corresponding sub-carriers. In someembodiments, a different set of gain factors is used for each frequencyhopping band. Additionally, a different gain factor may be applied tothe Inphase and Quadrature components of each sub-carrier. Duringtransmission, the frequency hopping band associated with the signal isdetermined and the appropriate set of gain factors is selected andapplied. The application of the gain factors compensates the spectraldistortion introduced by the DACs and various filters in the transmitsignal path. A substantially flat baseband signal similar to signal 430is thus obtained.

The synchronization sequences are often chosen for their autocorrelationand cross correlation properties rather than their spectral properties.As a result, the synchronization data sequence sometimes introducespeaks in the frequency spectrum, making the frequency spectrumsubstantially non-flat. FIG. 6A is a diagram illustrating the frequencyspectrum of a signal. In this example, the signal includes several peaks(such as peak 602) in its frequency spectrum. In some embodiments, thesignal is clipped at a level 604 to create a frequency spectrum that ismore even. FIG. 6B is a diagram illustrating the clipped frequencyspectrum. Details of the clipping process are discussed below.

FIG. 7 is a flowchart illustrating a frequency clipping processaccording to some embodiments. During process 700, frequency componentsof an input are limited to a predetermined clip level in order to reduceor eliminate the peaks and achieve a flat frequency spectrum. Theoriginal data sequence is first Fourier transformed to obtain itscomplex-valued spectral representation (702). A spectral component isthen selected (704). In this example, the spectral component (alsoreferred to as the Fourier coefficient) with the maximum amplitude ischosen. Next, the clip level is selected (705). The clip level, whichcontrols the flatness of the generated signal spectrum, is chosenrelative to the selected spectral amplitude in some embodiments. Theamplitudes of the spectral components are then clipped according to theclip level (706). In other words, spectral components with amplitudeexceeding the clip level are given a new amplitude value equal to theclip level. Other spectral components with amplitudes less than or equalto the clip level are unchanged. Finally, an inverse Fourier transformis applied to the clipped spectrum to transform the signal back to thetime domain (708).

The clipping operation can affect the auto-correlation andcross-correlation properties of the synchronization sequence. In someembodiments, a moderate clip level (for example, 3 dB below the maximumspectral amplitude) is chosen to achieve a substantial improvement ofthe spectral flatness with only a small impact on the performance of thereceiver. In some embodiments, the clip level is further reduced untilall the spectral components in the modified synchronization sequencehave approximately equal amplitude, thus creating a spectrum that issubstantially flat. In some cases, the clip level is set to a value lessthan or equal to the smallest spectral amplitude. In some embodiments,several outputs generated by using different clip levels are compared toselect an appropriate clip level that offers flat spectrum withoutsignificantly degrading the output sent to the receiver.

It is not necessary to perform the computations described in process 700for each data sequence during transmission. In some embodiments, similareffects are achieved by using pre-computed, modified synchronizationsequences that have reduced spectral peaks in the preamble waveform. Oneway to derive the modified synchronization sequences is to apply thecomputations of process 700 to different synchronization sequences andstore the results. FIG. 8 is a block diagram illustrating another OFDMtransmitter embodiment. Like components of transmitter 800 andtransmitter 300 perform like functions. In this example, modifiedsynchronization sequences are stored in a lookup table 802. When apreamble is to be generated, the modified synchronization sequence thatcorresponds to the preamble is retrieved and inserted into the signalstream. Other implementations are sometimes used in differentembodiments. For example, the preambles can be inserted prior to theIFFT operation. The frequency domain components may be clipped andbuffered before they are processed by the IFFT component.

In some embodiments, a random phase shifter that applies random orpseudorandom phase shifts to the OFDM symbols is used to randomize thesignal and reduce peaks in the frequency spectrum. The amount of phaseshift for each symbol may be determined according to a pseudo randomsequence or other predefined sequence. If desired, the sequence of phaseshifts can be reconstructed in the receiver, allowing the receiverremove the phase shift of each received OFDM symbol before other taskssuch as channel estimation, phase estimation and data demodulation arecarried out.

FIG. 9 is a block diagram illustrating another OFDM transmitterembodiment that implements the phase shift. In this example, a randomphase shifter 902 is used to introduce random or pseudo random phaseshifts to the OFDM symbols. In some embodiments, the phase shifts arelimited to multiples of 90° (i.e. the phase shifts are restricted to 0°,90°, 180°, 270°) so that the random phase shifter can be implemented viatwo basic operations: interchanging the I and Q signal components andreversing the sign of I and/or Q signal components. Although the randomphase shift is shown to take place prior to analog to digital conversionin this example, the phase shift operation may also be performedelsewhere in the transmitter. For example, the phase of the QPSK symbolsat the input of the IFFT may be shifted before the IFFT is applied.

The spectrum shaping techniques can be used in combination in someembodiments. For example, some transmitter embodiments include both amodified synchronization sequence lookup table for clipping preamblefrequency spectrum and a random phase shifter for performing phaseshift. Some transmitter embodiments use both a band gain control and asub-carrier amplitude control. FIG. 10 is a diagram illustrating atransmitter embodiment that includes several spectrum shapingcomponents. Transmitter 1000 shown in this example includes asub-carrier amplitude control 1002, a modified synchronization sequencelookup table 1004, a random phase shifter 1006 and a band gain control1008. One or more of these components may be active at the same time toshape the output signal to achieve a more uniform output spectrum.

FIG. 11 is a diagram illustrating an embodiment of a frequency hoppingtransmission that employs time spreading. In the example shown, OFDMsymbols 1100, 1102, 1104, 1106, 1108, and 1110 are part of a packet.Synchronization symbols for the packet are not illustrated and there maybe more OFDM symbols in addition to those illustrated. Using timespreading, two instances of each OFDM symbol are transmitted on the samehop band in this example. Time spreading is a process in which an OFDMsymbol is input and multiple instances of the OFDM symbol are output.Instances may be unmodified (an identical copy of the original OFDMsymbol) or may be modified. OFDM symbol A 1100 and OFDM symbol A′ 1102are transmitted on the hop band with center frequency f1, where OFDMsymbol A′ 1102 is a modified instance and OFDM symbol A 1100 is anunmodified instance. OFDM symbol B 1104 and OFDM symbol B 1106 aretransmitted on the hop band with center frequency f3 and OFDM symbol C1108 and OFDM symbol C′ 1110 are transmitted on the hop band with centerfrequency f2.

In some embodiments, all transmitted instances in a packet areunmodified. The two instances may be the output of a duplication blockthat outputs two identical instances for every OFDM symbol input. Forexample, OFDM symbols A, A, B, B, C, and C of a packet may betransmitted. In some embodiments, one or both of the transmittedinstances in a packet are a modification of the original OFDM symbol.For example, OFDM symbols A, A′, B, B′, C, and C′ of a packet may betransmitted. In another example, OFDM symbols A′, A″, B′, B″, C′, and C″of a packet may be transmitted, where OFDM symbol A″ is anothermodification of OFDM symbol A that is a different modification comparedto OFDM symbol A′. In some embodiments, a combination of methods isemployed in the same packet. In some embodiments, the method ofgenerating the two instances of an OFDM symbol is random.

In some embodiments, the frequency hopping scheme varies from thatillustrated. For example, there may be more or less than three hopbands. The sequence of hops may vary from that shown. In someembodiments, the hop band changes at a different rate than thatillustrated. For example, the hop band may change after every four OFDMsymbols transmitted instead of every two OFDM symbols. In someembodiments, frequency band hopping is not employed and allsynchronization symbols and OFDM symbols are transmitted on the sameband. For example, OFDM symbols A 1100, A′ 1102, B 1104, B 1106, C 1108,and C′ 1110 may be transmitted on band f1.

In some embodiments, the order of the OFDM symbols varies from thatillustrated. In some embodiments, the modified instance is transmittedbefore the unmodified instance. In some embodiments, the two instancesare not transmitted successively.

In some embodiments, the modification of the OFDM symbol is inversion.Some embodiments employ other modification techniques. For example, themodification may be swapping the I and Q signals or the modification maybe the complex conjugation of the OFDM symbol. Another examplemodification is phase shifting. A combination of methods may also beemployed.

If for some OFDM symbols both instances are transmitted on the same bandand the instances are different, the transmitted spectral shape may beflatter than if the instances are the same. Rather than having the twoinstances of an OFDM symbol repeat each other on the same band (and thusrepeat the same spectrum), two different instances may have differentspectrums and contribute to a flatter spectrum overall. For example, aprocess may be applied to select a subset of OFDM symbols in a packet.For the OFDM symbols not selected, the instances of each unselected OFDMsymbol are the same. In some embodiments, the instances are bothunmodified instances. For the subset of selected OFDM symbols, twodifferent instances of each selected OFDM symbol are transmitted. Insome embodiments, one instance is an unmodified instance and the otheris a modified instance of the original OFDM symbol. When the spectrum ismeasured (perhaps over multiple OFDM symbols or multiple packets) aflatter spectral shape is produced.

FIG. 12 is a flowchart illustrating an embodiment of a process fortransmitting a subset of OFDM symbols with different instances. In theexample shown, either a modified instance or an unmodified instance istransmitted after an unmodified instance of the OFDM symbol. A pseudorandom sequence (PRS) is obtained at 1200. For example, the pseudorandom sequence may be the sequence of pilot tones inserted into eachOFDM symbol. A pseudo random generator may also be used. In thisexample, the elements of the pseudo random sequence are either 1 or −1.An example of such a pseudo random sequence is [1 −1 −1 1 −1] where eachof the elements are generated using a random process. In someembodiments, a pseudo random number generator already included in thedesign is used to generate the random number multiple times. Logic isreused and die size and manufacturing costs are kept low.

In some embodiments, the elements in the random sequence take ondifferent values than those illustrated. The elements may take on morethan two values. In some embodiments, the values of the elements arediscrete values such as integer values. In some embodiments, theelements are continuous values.

The pseudo random sequence is shifted by K (PRS_(K)) at 1204. Forexample, if PRS=[1 −1 −1 1 −1] and K=2, then PRS_(K)=[−1 1 −1 −1 1 −1].At 1206 i is initialized to 0; i is used to track the current OFDMsymbol and the current index of the shifted pseudo random sequence. Thecurrent OFDM symbol (OFDM[i]) is transmitted at 1208.

At 1210 it is determined whether PRS_(K)[i mod n]=−1 where n is thelength of the pseudo random sequence. If the shifted pseudo randomsequence is equal to −1 then the current OFDM symbol is modified beforeit is transmitted at 1212. Control is then transferred to step 1216. Ifthe shifted pseudo random sequence is not equal to −1, then the currentOFDM symbol (OFDM[i]) is not modified before it is transmitted at 1214.Control is then transferred to step 1216.

At 1216, it is determined whether the current OFDM symbol is the lastOFDM symbol of the packet. If the current OFDM symbol is the last onethen the process ends. Otherwise, control is transferred to 1218 and iis incremented. Control is then transferred back to 1208 and the nextOFDM symbol (OFDM[i]) is transmitted.

FIG. 13 illustrates an embodiment of a shifted pseudo random sequenceused to select instances of OFDM symbols to modify. In the exampleshown, pseudo random sequence 1302 is shifted by K=2 to created shiftedpseudo random sequence 1304. Shifted pseudo random sequence 1304 is usedto determine which input components of input signal 1300 to modify. Theinput signal may represent a packet and the input components are theOFDM symbols of the packet. Output signal 1304 represents the sequenceof time spread instances transmitted.

Shifted pseudo random sequence 1304 is used to determine whether outputsignal 1306 includes a modified instance or an unmodified instance forthe second instance of each OFDM symbol. The first input element ininput signal 1300, OFDM symbol A, is copied to output signal 1306. Sincethe first element in shifted pseudo random sequence 1304 is −1, amodified instance (A′) is copied to output signal 1306. Otherwise anunmodified instance is copied. This process repeats for the rest of theinput elements. If there are more OFDM symbols in input signal 1300 thanelements in shifted pseudo random sequence 1304 the index wraps to thebeginning of the shifted pseudo random sequence 1304. Thus, both OFDMsymbol A and OFDM symbol F use the first element in shifted pseudorandom sequence 1304, which is −1.

FIG. 14A illustrates an embodiment of a transmitter which implementsspectral shaping using time spreading in the time domain. In the exampleshown, corresponding modules perform the same functions as thosedescribed in FIG. 3. Pilot insertion block 1400 and time spreading block1402 perform spectral shaping using time spreading. Time spreading block1402 is after IFFT 1408 and processes time domain signals. For everyOFDM symbol that is passed to time spreading block 1402, two instancesare output to DACs 1404 and 1406. The first instance output by timespreading block 1402 may be an unmodified copy of the input OFDM symbol.The second instance is either a modified instance or an unmodifiedinstance of the input OFDM symbol. The pilot sequence from pilotinsertion block 1400 is used as a pseudo random sequence and it (or ashifted version of it) is used to determine which OFDM symbols tomodify. Time spreading block 1402 decides which OFDM symbols to modifyusing the pilot sequence and performs the appropriate modification orduplication.

FIG. 14B illustrates an embodiment of a transmitter which implementsspectral shaping using time spreading in the frequency domain. In theexample illustrated, corresponding modules perform the same functions asthose described in FIG. 3. Time spreading block 1452 is before IFFT 1454and processes frequency domain signals rather than time domain signals.Time spreading block 1452 outputs two instances of an OFDM symbol forevery OFDM symbol passed to it. The first instance is an unmodifiedinstance and the second is either a modified instance or an unmodifiedinstance. In this example, time spreading block 1452 is coupled to pilotinsertion block 1450. The pilot sequence from pilot insertion block 1450may be shifted and used to decide which OFDM symbols to modify. Timespreading block 1452 performs this decision making and the appropriatecopying/modifying of OFDM symbols passed to it.

The illustrated placement of time spreading block 1402 in thetransmitter may consume less power compared to time spreading block1452. Since time spreading block 1452 is before IFFT 1454, IFFT 1454must process both instances of each OFDM symbol generated. IFFT 1408,which precedes time spreading block 1402, does not process bothinstances. This results in less power consumed by the transmitter to runIFFT 1408 using time spreading block 1402.

In some embodiments, time spreading is performed at other points withinthe transmitter than those illustrated. Design complexity, die size, andpower consumption may be considered when deciding where in thetransmitter block diagram to perform time spreading. In someembodiments, it may be simpler to combine spectral shaping using timespreading with other modules. In some embodiments, the time spreadingblock is implemented as multiple modules. For example, a first modulemay duplicate each OFDM symbol passed to it. A subsequent block maydecide which duplicate OFDM symbols to modify and performs themodification.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

1. A method of shaping an orthogonal frequency division multiplexing(OFDM) signal spectrum of a transmitted signal, comprising: receiving aninput signal including an input component; generating a first instanceof the input component; determining that a second instance of the inputcomponent is to be different than the first instance; generating thesecond instance of the input component that is different from the firstinstance; and generating an output signal to be transmitted, wherein theoutput signal includes the first instance and the second instance.
 2. Amethod as recited in claim 1, wherein the input component is an OFDMsymbol.
 3. A method as recited in claim 1, wherein the first instanceand the second instance is are transmitted on a same transmission band.4. A method as recited in claim 1 further comprising transmitting theoutput signal using frequency band hopping.
 5. A method as recited inclaim 1, wherein the input signal includes a plurality of inputcomponents and all second instances of the plurality of input componentsare determined to be different than their corresponding first instance.6. A method as recited in claim 1, wherein generating the secondinstance of the input component includes inversion.
 7. A method asrecited in claim 1, wherein generating the second instance of the inputcomponent includes swapping I and Q elements.
 8. A method as recited inclaim 1, wherein generating the second instance of the input componentincludes complex conjugation.
 9. A method as recited in claim 1, whereingenerating the second instance of the input component includes phaseshifting.
 10. A method as recited in claim 1, wherein generating thesecond instance of the input component includes using a time domainrepresentation.
 11. A method as recited in claim 1 further comprisinggenerating a pseudo random number, wherein the pseudo random number isused in determining that the second instance of the input component isto be different than the first instance.
 12. A method as recited inclaim 1, wherein: the input signal includes a plurality of inputcomponents; and a pseudo random sequence is used in determining which ofthe second instances of the plurality of input components are to bedifferent than their corresponding first instance.
 13. A method asrecited in claim 1, wherein: the input signal includes a plurality ofinput components; a pseudo random sequence is used in determining whichof the second instances of the plurality of input components are to bedifferent than their corresponding first instance; and in the eventthere are more input components than elements in the pseudo randomsequence, the pseudo random sequence is reused.
 14. A method as recitedin claim 1, wherein: the input signal includes a plurality of inputcomponents; and a sequence used in the generation of pilots is used indetermining which of the second instances of the plurality of inputcomponents are to be different than their corresponding first instance.15. A system for shaping an orthogonal frequency division multiplexing(OFDM) signal spectrum of a transmitted signal, comprising: an interfaceconfigured to receive an input signal including an input component; anda signal processor configured to: generate a first instance of the inputcomponent; determine that a second instance of the input component is tobe different than the first instance; generate the second instance ofthe input component that is different from the first instance; andgenerate an output signal to be transmitted, wherein the output signalincludes the first instance and the second instance.
 16. A system asrecited in claim 15, wherein the first instance and the second instanceare transmitted on a same transmission band.
 17. A system as recited inclaim 15, wherein generating the second instance of the input componentincludes inversion.
 18. A system as recited in claim 15, wherein: theinput signal includes a plurality of input components; and a pseudorandom sequence is used in determining which of the second instances ofthe plurality of input components are to be different than theircorresponding first instance.
 19. A computer program product for shapingan orthogonal frequency division multiplexing (OFDM) signal spectrum ofa transmitted signal, the computer program product being embodied in acomputer readable medium and comprising computer instructions for:receiving an input signal including an input component; generating afirst instance of the input component; determining that a secondinstance of the input component is to be different than the firstinstance; generating the second instance of the input component that isdifferent from the first instance; and generating an output signal to betransmitted, wherein the output signal includes the first instance andthe second instance.
 20. A computer program product as recited in claim19, wherein the first instance and the second instance are transmittedon a same transmission band.
 21. A computer program product as recitedin claim 19, wherein generating the second instance of the inputcomponent includes inversion.
 22. A computer program product as recitedin claim 19, wherein: the input signal includes a plurality of inputcomponents; and a pseudo random sequence is used in determining which ofthe second instances of the plurality of input components are to bedifferent than their corresponding first instance.